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Google has said its scanning for child abuse images on Gmail and other services does not extend to searching for evidence of other crimes.

US police recently arrested a registered sex offender after Google notified the authorities about illegal photos of children that were allegedly found in the 41-year-old's Gmail account.

The tip-off gave police in Houston enough to go on to obtain a warrant to arrest the suspect, identified in US reports as John Henry Skillern, and seize his computers and associated kit. Police alleged a subsequent forensic examination revealed a store of child abuse images on a tablet and phone owned by Skillern.

Uncovering criminals who are active in exchanging images of child abuse is an obvious public good, but Google's role in this case raises questions about just how closely the search engine giant is scrutinising our webmail for evidence of criminal activity.

In a statement, Google outlined its use of automated image scanning technology to fight child abuse online, an established practice though one not previously connected to Gmail as such. It added that it is not looking for evidence of more general criminality that might be gleaned from scanning customers' webmail accounts.

Sadly all Internet companies have to deal with child sexual abuse. It’s why Google actively removes illegal imagery from our services – including search and Gmail – and immediately reports any abuse to the National Center for Missing and Exploited Children, which collaborates with law enforcement.

This evidence is regularly used to convict criminals. Each child sexual abuse image is given a unique digital fingerprint which enables our systems to identify those pictures, including in Gmail. It is important to remember that we use this technology to identify child sexual abuse imagery, not email content that could be associated with general criminal activity (for example using email to plot a burglary).

Gmail's terms of service already indicate that the company is analysing Gmail for both targeted ads and security, a category we now know extends to searches for child abuse images and (presumably) other illegal pornography.

Google uses MD5 hashes of known malware samples to scan for malicious code in webmail. It's not clear how it searches for images of child abuse in webmail though something along the same lines is a strong possibility. ®